Apparatus and method for electronic circuit protection

ABSTRACT

Apparatus and methods for electronic circuit protection are disclosed. In one embodiment, an apparatus comprises a well having an emitter and a collector region. The well has a doping of a first type, and the emitter and collector regions have a doping of a second type. The emitter region, well, and collector region are configured to operate as an emitter, base, and collector for a first transistor, respectively. The collector region is spaced away from the emitter region to define a spacing. A first spacer and a second spacer are positioned adjacent the well between the emitter and the collector. A conductive plate is positioned adjacent the well and between the first spacer and the second spacer, and a doping adjacent the first spacer, the second spacer, and the plate consists essentially of the first type.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/797,463, filed Jun. 9, 2010, entitled “APPARATUS AND METHOD FORELECTRONIC SYSTEMS RELIABILITY”, the entire disclosure of which ishereby incorporated herein by reference. This application is alsorelated to U.S. application Ser. No. 12/797,461, filed Jun. 9, 2010,entitled “APPARATUS AND METHOD FOR PROTECTING ELECTRONIC CIRCUITS”, nowU.S. Pat. No. 8,368,116, issued Feb. 5, 2013, the entire disclosure ofwhich is hereby incorporated herein by reference.

BACKGROUND

1. Field

Embodiments of the invention relate to electronic systems, and moreparticularly, to protection circuits for electronic systems.

2. Description of the Related Technology

Certain electronic systems can be exposed to a transient signal event,or an electrical signal of a relatively short duration having rapidlychanging voltage and high power. Transient signal events can include,for example, electrostatic discharge (ESD) events arising from theabrupt release of charge from an object or person to an electronicsystem.

Transient signal events can damage integrated circuits (ICs) inside anelectronic system due to overvoltage conditions and/or high levels ofpower dissipation over relatively small areas of the ICs. High powerdissipation can increase IC temperature, and can lead to numerousproblems, such as gate oxide punch-through, junction damage, metaldamage, and surface charge accumulation. Moreover, transient signalevents can induce latch-up (in other words, inadvertent creation of alow-impedance path), thereby disrupting the functioning of the IC andpotentially causing permanent damage to the IC. Thus, there is a need toprovide an IC with protection from such transient signal events.

SUMMARY

In one embodiment, an apparatus for providing protection from transientelectrical events comprises an integrated circuit, a pad on a surface ofthe integrated circuit, and a configurable protection circuit within theintegrated circuit. The configurable protection circuit is electricallyconnected to the pad. Additionally, the configurable protection circuitcomprises a plurality of subcircuits arranged in a cascade, andselection of one or more of the plurality of the subcircuits foroperation determines at least one of a holding voltage or a triggervoltage of the configurable protection circuit.

In another embodiment, a method for providing protection from transientsignals comprises providing an integrated circuit having a pad on asurface of the integrated circuit and having a configurable protectioncircuit comprising a plurality of subcircuits. The method furthercomprises selecting one or more of the plurality of the subcircuits foroperation in a cascade, wherein selecting the one or more of theplurality of the subcircuits for operation determines at least one of aholding voltage or a trigger voltage of the configurable protectioncircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of one example of an electronicsystem including integrated circuits (ICs).

FIG. 2 is a schematic block diagram of an integrated circuit includingpad circuits according to some embodiments.

FIG. 3A is a graph of one example of pad circuit current versustransient signal voltage.

FIG. 3B is a graph of another example of pad circuit current versustransient signal voltage.

FIG. 4A is a schematic block diagram of a pad circuit in accordance withone embodiment.

FIG. 4B is a schematic block diagram of a pad circuit in accordance withanother embodiment.

FIG. 5A is a circuit diagram illustrating a pad circuit building blockin accordance with one embodiment.

FIG. 5B is a circuit diagram illustrating a pad circuit building blockin accordance with another embodiment.

FIG. 5C is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment.

FIG. 6A is a cross section of a conventional NMOS transistor having alightly doped drain (LDD) structure.

FIG. 6B is a cross section of an NPN bipolar transistor in accordancewith one embodiment.

FIG. 6C is a cross section of a PNP bipolar transistor in accordancewith another embodiment.

FIG. 7A is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment.

FIG. 7B is a cross section of one implementation of the pad circuitbuilding block of FIG. 7A.

FIG. 8A is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment.

FIG. 8B is a cross section of one implementation of the pad circuitbuilding block of FIG. 8A.

FIG. 9A is a schematic block diagram of a pad circuit according to afirst embodiment.

FIG. 9B is a circuit diagram of the pad circuit of FIG. 9A.

FIG. 10A is a schematic block diagram of a pad circuit according to asecond embodiment.

FIG. 10B is a circuit diagram of the pad circuit of FIG. 10A.

FIG. 11A is a schematic block diagram of a pad circuit according to athird embodiment.

FIG. 11B is a circuit diagram of the pad circuit of FIG. 11A.

FIG. 12A is a schematic block diagram of a pad circuit according to afourth embodiment.

FIG. 12B is a circuit diagram of the pad circuit of FIG. 12A.

FIG. 13A is a schematic block diagram of a pad circuit according to afifth embodiment.

FIG. 13B is a circuit diagram of the pad circuit of FIG. 13A.

FIG. 14A is a schematic block diagram of a pad circuit according to asixth embodiment.

FIG. 14B is a circuit diagram of the pad circuit of FIG. 14B.

FIG. 15 is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment.

FIG. 16A is a schematic block diagram of a pad circuit according to aseventh embodiment.

FIG. 16B is a circuit diagram of the pad circuit of FIG. 16A.

FIG. 17A is a perspective view of one implementation of the pad circuitof FIG. 12B.

FIG. 17B is a cross section of the pad circuit of FIG. 17A taken alongthe line 17B-17B.

FIG. 17C is a cross section of the pad circuit of FIG. 17A taken alongthe line 17C-17C.

FIG. 17D is a cross section of the pad circuit of FIG. 17A taken alongthe line 17D-17D.

FIG. 17E is a top plan view of the active and polysilicon layers of thepad circuit of FIG. 17A.

FIG. 17F is a top plan view of the contact and first metal layers of thepad circuit of FIG. 17A.

FIG. 17G is a top plan view of the first metal layer and first via layerof the pad circuit of FIG. 17A.

FIG. 17H is a top plan view of the second metal layer and second vialayer of the pad circuit of FIG. 17A.

FIG. 17I is a top plan view of the third metal layer of the pad circuitof FIG. 17A.

FIG. 18A is a perspective view of one implementation of the pad circuitof FIG. 11B.

FIG. 18B is a cross section of the pad circuit of FIG. 18A taken alongthe line 18B-18B.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of certain embodiments presentsvarious descriptions of specific embodiments of the invention. However,the invention can be embodied in a multitude of different ways asdefined and covered by the claims. In this description, reference ismade to the drawings where like reference numerals indicate identical orfunctionally similar elements.

Electronic systems are typically configured to protect circuits orcomponents therein from transient signal events. Furthermore, to helpassure that an electronic system is reliable, manufacturers can test theelectronic system under defined stress conditions, which can bedescribed by standards set by various organizations, such as the JointElectronic Device Engineering Council (JEDEC), the InternationalElectrotechnical Commission (IEC), and the Automotive EngineeringCouncil (AEC). The standards can cover a wide range of transient signalevents, including ESD events.

Electronic circuit reliability can be improved by coupling padprotection circuits to the pads of an IC for transient signalprotection. The pad circuits can be configured to maintain the voltagelevel at the pad within a predefined safe range. However, it can bedifficult to provide pad circuits that meet reliability and performancerequirements with low manufacturing cost and a relatively small circuitarea.

An integrated circuit (IC) can have many pads, and different pads can beexposed to different voltage domains. Each voltage domain can havedifferent performance and reliability requirements. For example, eachvoltage domain can have a different minimum operating voltage, maximumoperating voltage, and constraint on leakage current. There is a needfor providing IC protection pads operating over a multitude of voltagedomains to enhance electronic circuit reliability for ICs in a simpleand cost-effective manner.

Overview of Electronic Systems

FIG. 1 is a schematic block diagram of an electronic system 10, whichcan include one or more pad circuits according to an embodiment of theinvention. The illustrated electronic system 10 includes a first IC 1, asecond IC 2, and pins 4, 5, 6. As illustrated in FIG. 1, the pin 4 iselectrically connected to the first IC 1 by a connection 7. The pin 5 iselectrically connected to the second IC 2 by a connection 8. Theelectronic system 10 can also include pins electrically connected toboth the first and second ICs 1, 2. For example, the illustrated pin 6is electrically connected to the first and second ICs 1, 2 by aconnection 9. Additionally, the first and second ICs 1, 2 can beelectrically connected to one another by one or more connectionsinternal to the electronic system 10, such as by connections 11 and 12.The first and second ICs 1, 2 can be exposed to user contact via, forexample, the pins 4, 5, 6. The user contact can be through a relativelylow-impedance connection.

The first and second ICs 1, 2 can be exposed to transient signal events,such as ESD events, which can cause IC damage and induce latch-up. Forexample, the connection 11 can receive a device-level transient signalevent 14, and/or the pin 6 can receive a system-level transient signalevent 16. The transient signal events 14, 16 can travel along theconnections 11, 9, respectively, and can be received at the pads of thefirst and second ICs 1, 2.

In some embodiments, the first and second ICs 1, 2 can include pads, andcan be provided with pad circuits configured to ensure reliability ofthe ICs by maintaining the voltage level at the pads within a selectedrange, which can vary from pad to pad. For example, either or both ofthe first and second ICs 1, 2 can include one or more pads configured tooperate over a multitude of voltage domains or current bias conditions,each having varying performance and reliability requirements.

Overview of Power Management ICs

In some embodiments, one or more pad circuits can be employed in an IC,such as the first IC 1 of FIG. 1, and can be configured to providetransient signal protection to one or more internal circuits of the IC.The pad circuit can be configured to divert a current associated with atransient signal event received on a pad of the IC to other nodes orpads of the IC, thereby providing transient signal protection, as willbe described in further detail below. The current can be shunted from,for example, a low-impedance output pad, a high-impedance input pad, ora low-impedance power or ground pad, to a low impedance pad or node ofthe IC. When no transient signal event is present, the pad circuit canremain in a high-impedance/low-leakage state, thereby reducing orminimizing static power dissipation resulting from leakage current andimproving the operation of leakage sensitive circuitry, as will bedescribed in detail below.

In other embodiments, one or more pad circuits can be provided in asingle IC (for example, the first IC 1 of FIG. 1), and can be configuredto provide transient signal protection for another component (forexample, the second IC 2 of FIG. 1). The first IC 1 can be physicallyseparated from the second IC 2, or it can be encapsulated in a commonpackage with the second IC 2. In such embodiments, one or more padcircuits can be placed in a stand-alone IC, in a common package forsystem-on-a-package applications, or integrated with an IC in a commonsemiconductor substrate for system-on-a-chip applications.

FIG. 2 is a schematic block diagram of one example of an integratedcircuit (IC) including pad circuits according to some embodiments. TheIC 20 can be a power management IC, which can include, for example, padcircuits 22 a-22 p, a pad controller 23, comparators 27 a-27 h, amultiplexer 30, first and second OR gates 31 a, 31 b, an output logic32, a clear logic 33, a voltage reference circuit 35, a timer 39, andpads 42 a-42 p. The power management IC 20 can be included in anelectronic system, such as the electronic system 10 of FIG. 1, and canbe, for example, the first IC 1 or the second IC 2. Depending on adesign specification, not all of the illustrated components arenecessary. For example, skilled artisans will appreciate that the padcontroller 23 need not be included, that the power management IC 20 canbe modified to monitor more or fewer voltage domains, and that the powermanagement IC 20 can have more extensive or less extensivefunctionality.

Furthermore, although the pad circuits are illustrated in the context ofthe power management IC 20, the pad circuits can be employed in a widearray of ICs and other electronics having pads configured to operateover a multitude of voltage domains or current bias conditions.

The power management IC 20 can be configured to simultaneously monitormultiple voltage domains for overvoltage and undervoltage conditions, aswill be described below. For example, the power management IC 20 cangenerate an overvoltage signal coupled to the pad 42 i (OVERVOLTAGE),which can indicate whether or not an overvoltage condition is detectedon any of the pads 42 a-42 d (VH1, VH2, VH3, and VH4, respectively).Additionally, the power management IC 20 can generate an undervoltagesignal coupled to the pad 42 j (UNDERVOLTAGE), which can indicatewhether or not an undervoltage condition is detected on any of the pads42 e-42 h (VL1, VL2, VL3, and VL4, respectively). Although theillustrated power management IC 20 is configured to monitor up to fourvoltage domains, skilled artisans will appreciate that this choice ismerely illustrative, and that alternate embodiments of the powermanagement IC 20 can be configured to be able to monitor more or fewervoltage domains, as well as to feature more extensive or less extensivefunctionality.

The power management IC 20 can aid in the integration and bias of ICsand other components of the electronic system 10. The power managementIC 20 can also detect overvoltage conditions and/or undervoltageconditions which can endanger the proper operation of the electronicsystem 10. Additionally, the power management IC 20 can aid in reducingpower consumption by detecting overvoltage conditions which canundesirably increase power consumption.

The power management IC 20 can be subject to stringent performance anddesign requirements. For example, the power management IC 20 can besubject to relatively tight constraints on leakage current in order toreduce static power dissipation and to improve performance forleakage-sensitive circuitry, as will be described below. Additionally,the power management IC 20 can be used to interact with multiple voltagedomains, and thus should be able to handle relatively high input andoutput voltages without latching-up or sustaining physical damage.Moreover, there can be stringent requirements regarding the expense ofthe design and manufacture of the power management IC 20. Furthermore,in certain embodiments, configurability of the performance and designparameters of the power management IC 20 can be desirable, therebypermitting the power management IC 20 to be employed in a vast array ofelectronic systems and applications.

Each of the comparators 27 a-27 h can monitor an overvoltage orundervoltage condition of a voltage domain. This can be accomplished byproviding a voltage from a voltage domain to a comparator. For example,a resistor divider (not shown in FIG. 2) having a series of resistorscan be placed between a voltage supply of a voltage domain and a voltagereference, such as ground. A voltage can be tapped between the series ofresistors and can be provided to a pad of the power management IC 20,such as, for example, the pad 42 a (VH1). The voltage received at thepad 42 a can be provided to the comparator 27 a, which in turn cancompare the voltage received from the pad 42 a to a threshold voltageVx. In one embodiment, the threshold voltage Vx is selected to be about500 mV. By selecting the voltage provided to the pad 42 a (for example,by selecting the number and magnitude of the resistors in the divider),the output of the comparator 27 a can be configured to change when thevoltage supply of a voltage domain exceeds a selected value. Likewise,by selecting the voltage provided to the pad 42 e in a similar manner,the output of the comparator 27 e can be configured to change when thesupply of a voltage domain falls below a selected value.

As described above, the voltage provided to the pads 42 a-42 h can beprovided from a resistor divider. The impedance of the resistors in theresistor divider can be relatively large (for example, tens ofMega-Ohms) so as to minimize system-level static power consumption.Thus, the accuracy of the resistor divider can be sensitive to theleakage of the pads 42 a-42 h, and there can be stringent performancerequirements on the leakage current of the pads 42 a-42 h.

The first OR gate 31 a can determine if one or more of the comparatorscoupled to its inputs indicate that an overvoltage condition has beendetected. Likewise, the second OR gate 31 b can determine if one or moreof the comparators coupled to its inputs indicate that an undervoltagecondition has been detected. In the illustrated embodiment, the outputsof comparators 27 a, 27 b are provided to the first OR gate 31 a, whilethe outputs of the comparators 27 e, 27 f are provided to the second ORgate 31 b.

Additionally, the first and second OR gates 31 a, 31 b can each receivesignals from the multiplexer 30. The multiplexer 30 can allowovervoltage and undervoltage detection to be performed on voltagedomains having a negative polarity with respect to the voltage receivedon the ground pad 42 o (GND), such that overvoltage and undervoltagerelate to magnitudes or absolute values of voltage. In particular, themultiplexer 30 can select which comparator signals are provided to thefirst and second OR gates 31 a, 31 b in response to a select controlsignal received from the pad 42 p (SEL). For example, the multiplexer 30can be configured to selectively provide the first OR gate 31 a with theoutput of the comparator 27 c or the comparator 27 g, and the output ofthe comparator 27 d or the comparator 27 h, based on a state of theselect control signal received from the pad 42 p (SEL). Likewise, themultiplexer 30 can be configured to selectively provide the second ORgate 31 b with the output of the comparator 27 c or the comparator 27 g,and the output of the comparator 27 d or the comparator 27 h, based on astate of the select control signal received from the pad 42 p (SEL). Byselecting which comparator outputs are provided to the first and secondOR gates 31 a, 31 b, overvoltage and undervoltage detection can beperformed on the voltages on the pads 42 c, 42 d and 42 g, 42 h, evenfor voltage domains having a negative polarity with respect to ground.The multiplexer 30 can be implemented with logic gates, with 3-stategates, or the like.

The output logic 32 can control the state of the pad 42 i (OVERVOLTAGE)and the pad 42 j (UNDERVOLTAGE). For example, the output logic 32 canindicate that an overvoltage or undervoltage condition has been detectedbased at least in part on the outputs of the first and second OR gates31 a, 31 b. The output logic 32 can signal the detection of anovervoltage or undervoltage condition for a duration exceeding the timethat the first or second OR gates 31 a, 31 b indicates that anovervoltage or undervoltage condition has been detected. For example,the output logic 32 can receive a signal from the timer 39, which canindicate the duration that the overvoltage or undervoltage conditionshould be asserted. The timer 39 can be electrically connected to thepad 42 m (TIMER) and can be configured to have a drive strength andcorresponding drive resistance. The pad 42 m can be electricallyconnected to an external capacitor, which can have a variablecapacitance to establish an RC time constant for determining the resetdelay of the timer 39.

The output logic 32 can also be configured to communicate with the clearlogic 33. The clear logic 33 can receive a clear control signal from pad42 k (CLEAR). In response to the clear control signal, the output logic32 can reset the state of the pads 42 i (OVERVOLTAGE) and 42 j(UNDERVOLTAGE) to indicate that no overvoltage or undervoltage conditionhas been detected.

The power management IC 20 can also provide an output reference voltageon pad 42 l (V_(REF)). This voltage can be selected to be, for example,about 1 V. The output voltage reference can be used by other componentsof the electronic system in which the power management IC 20 isimplemented (for example, the electronic system 10 of FIG. 1). Forexample, the reference voltage can be provided as a reference voltage toone end of a resistor divider configured to provide a voltage to thepads 42 a-42 h for overvoltage or undervoltage detection.

As described above, the power management IC 20 can be configured tomonitor multiple voltage domains, for example, four voltage domains forovervoltage and undervoltage conditions. Each of the voltage domains canhave the same or different operating conditions and parameters.Additionally, the power management IC 20 can include a multitude ofoutput pads, such as the pad 42 i for indicating the detection of anovervoltage condition, the pad 42 j for indicating the detection of anundervoltage condition, the pad 42 p for providing the output voltagereference. The power management IC 20 can also include control pads,such as the pad 42 p (SEL), the pad 42 k (CLEAR), and the pad 42 m(TIMER). Furthermore, the power management IC 20 can include the powerpad 42 n (Vcc) and the ground pad 42 o (GND).

In some embodiments, the electronic system (for example, the electronicsystem 10 of FIG. 1) having the pads 42 a-42 p can have differentrequirements for minimum operating voltage, maximum operating voltage,and leakage current for each of the pads 42 a-42 p. Thus, each of thepads 42 a-42 p described above can have different performance and designrequirements. In order to meet reliability requirements across a widevariety of applications, it can be desirable that one or more of thepads 42 a-42 p have a pad circuit configured to protect the powermanagement IC 20 from overvoltage conditions and latch-up. Furthermore,it can be desirable that each pad circuit 22 a-22 p is configurable tooperate with different reliability and performance parameters, forexample, by changing only metal layers during back-end processing, or byusing the pad controller 23 after fabrication. This can advantageouslypermit the pad circuits 22 a-22 p to be configurable for a particularapplication without requiring a redesign of the power management IC 20.

FIG. 3A illustrates a graph 60 of one example of pad circuit currentversus transient signal voltage. As described above, it can be desirablefor each pad circuit 42 a-42 p to be configured to maintain the voltagelevel at the pad within a predefined safe range. Thus, the pad circuitcan shunt a large portion of the current associated with the transientsignal event before the voltage of the transient signal V_(TRANSIENT)reaches a voltage V_(FAILURE) that can cause damage to the powermanagement IC 20. Additionally, the pad circuit can conduct a relativelylow current at the normal operating voltage V_(OPERATING), therebyminimizing static power dissipation resulting from the leakage currentI_(LEAKAGE) and improving the performance of leakage sensitivecircuitry, such a resistor divider.

Furthermore, as shown in the graph 60, the pad circuit can transitionfrom a high-impedance state Z_(H) to a low-impedance state Z_(L) whenthe voltage of the transient signal V_(TRANSIENT) reaches the voltageV_(TRIGGER). Thereafter, the pad circuit can shunt a large current overa wide range of transient signal voltage levels. The pad circuit canremain in the low-impedance state Z_(L) as long as the transient signalvoltage level is above a holding voltage V_(HOLDING) and the rate ofvoltage change is in the range of normal frequency operating conditions,rather than in the range of high frequency conditions and relativelyfast rise and fall times which can be associated with a transient signalevent. In certain embodiments, it can be desirable for the holdingvoltage V_(HOLDING) to be above the operating voltage V_(OPERATION) sothat the pad circuit does not remain in the low-impedance state Z_(L)after passage of the transient signal event and a return to normaloperating voltage levels.

FIG. 3B is a graph 62 of another example of pad circuit current versustransient signal voltage. As shown in FIG. 3B, a pad circuit cantransition from a high-impedance state Z_(H) to a low-impedance stateZ_(L) when the voltage of the transient signal V_(TRANSIENT) reaches thevoltage V_(TRIGGER). Thereafter, the pad circuit can shunt a largecurrent over a wide range of transient signal voltage levels. The padcircuit can remain in the low-impedance state Z_(L) as long as thetransient signal voltage level is above a holding voltage V_(HOLDING).It can be desirable for the holding voltage V_(HOLDING) to be below theoperating voltage V_(OPERATION) in order to provide enhanced protectionagainst transient signal events and to reduce the circuit area needed toprovide a desired pad shunting current. This technique can be employed,for example, in embodiments in which the holding current I_(HOLDING)exceeds the maximum current the pad can supply when biased at normaloperating voltage levels. Thus, in certain embodiments, the pad circuitneed not remain in the low-impedance state Z_(L) after passage of thetransient signal event and a return to normal operating voltage levels,even when V_(OPERATION) exceeds V_(HOLDING), because the pad may not beable to supply a sufficient holding current I_(HOLDING) to retain thepad circuit in the low-impedance state Z_(L).

As described above, the operating and reliability parameters of a padcircuit can vary widely, depending on a particular application. Forpurposes of illustration only, one particular electronic system can havethe characteristics shown in Table 1 below for selected pads of FIG. 2.

TABLE 1 V_(OPERATION) V_(HOLDING) V_(TRIGGER) I_(LEAKAGE) Pad Min MaxMin Max Min Max Min Max VH1 0 V  8 V 9 V 13 V 16 V 20 V 0 nA 15 nA VH2 0V  8 V 6 V 10 V 16 V 20 V 0 nA 15 nA VH3 0 V  8 V 3 V  7 V 16 V 20 V 0nA 15 nA VH4 0 V 16 V 6 V 10 V 24 V 30 V 0 nA 15 nA Vcc 18 V  20 V 22 V 24 V 24 V 30 V 0 nA 10 nA OVERVOLTAGE 0 V 16 V 14 V  18 V 24 V 30 V 0 nA15 nA UNDERVOLTAGE 0 V 16 V 8 V 12 V 24 V 30 V 0 nA 15 nA

There is a need for pad circuits which can be configured to meet theperformance and design parameters of an electronic circuit or IC (suchas the power management IC 20 of FIG. 2) required for a particularapplication. Furthermore, in certain embodiments, there is a need forpad circuits which can operate with different reliability andperformance parameters, for example, by changing only metal layers, orby configuring the power management IC 20 post-fabrication by selectingthe setting of a pad controller 23. This can advantageously permit padcircuits 42 a-42 p to be configured for a particular application withoutrequiring a redesign of the power management IC 20. The pad controller23 can employ metal or poly fuses to control the operation of an ESDtolerant switch, as will be described in further detail below.

IC Pad Circuits for Protection from Transient Signal Event

FIG. 4A is a schematic block diagram of a pad circuit 22 according to anembodiment of the invention. The illustrated pad circuit 22 includes afirst building block 72, a second building block 74, and a thirdbuilding block 76. The first, second, and third building blocks 72, 74,76 can be connected end-to-end in a cascade configuration between a pad42 and a node 82, and can be subcircuits of the pad circuit 22.Additional or fewer building blocks can be included in the cascade toachieve the desired reliability and performance parameters, as will bedescribed in further detail below. The pad circuit 22 can be, forexample, any of the pad circuits 22 a-22 p shown in FIG. 2, and the pad42 can be any of the pads 42 a-42 p, including, for example,low-impedance output pads, high-impedance input pads, and low-impedancepower pads. The node 82 can be, for example, a low impedance node or padof the power management IC 20 configured to handle a relatively largeshunted current.

The building blocks 72, 74, 76 can form a pad circuit that hascharacteristics shown in FIG. 3A or 3B. In one embodiment, the first,second and third building blocks 72, 74, 76 can be selected from avariety of types, such as a variety of electrically isolated clampstructures, so as to achieve the desired performance and reliabilityparameters for the pad circuit 22. For example, a first type of buildingblock (Type A) can have a holding voltage V_(H) _(—) _(A) and a triggervoltage V_(T) _(—) _(A). A second type of building block (Type B) canhave, for example, a trigger voltage V_(T) _(—) _(B) and a holdingvoltage V_(H) _(—) _(B). By arranging additional or fewer of each typeof building block, the overall holding voltage and trigger voltage ofembodiments of the pad circuit 22 can be selectively varied. As will bedescribed below, the building block types can be selected such that,when combining i number of Type A building blocks and j number of Type Bbuilding blocks in a cascade configuration, the pad circuit 22 can havea trigger voltage V_(TRIGGER) roughly equal to about i*V_(T) _(—)_(A)+j*V_(T) _(—) _(B), and a holding voltage V_(HOLDING) roughly equalto about i*V_(H) _(—) _(A)+j*V_(H) _(—) _(B). Thus, by selecting thetype and/or number of building blocks employed after manufacturing,and/or selecting the value of V_(H) _(—) _(A), V_(H) _(—) _(B), V_(T)_(—) _(A) and V_(T) _(—) _(B) during design of the building blocks, ascalable family of pad circuit embodiments can be created which can beadapted for a multitude of electronic systems and applications.

The design cost associated with designing the pad circuits can bereduced as compared to, for example, an approach in which differentdiode, bipolar, silicon controlled rectifier, and/or MOS devices areemployed to achieve the reliability and performance requirements neededfor each pad circuit. Moreover, in one embodiment, a first buildingblock is placed below the pad and additional building blocks are placedin the vicinity of the pad. During back-end fabrication (for example,fabrication of metal layers), building blocks can be included in acascade configuration with the first building block. Thus, each padcircuit 22 can be configured for a particular electronic system orapplication by changing the metal layers to control the building blockconfiguration, as will be described below.

FIG. 4B is a schematic block diagram of a pad circuit in accordance withone embodiment. The illustrated pad circuit 22 includes a first buildingblock 72, a second building block 74, and a third building block 76. Thefirst, second, and third building blocks 72, 74, 76 can be connectedend-to-end in a cascade configuration between a pad 42 and a node 82.Additional or fewer building blocks and blocks of a variety of types canbe included in the cascade, as described earlier in connection with FIG.4A.

Additionally, as illustrated in FIG. 4B, the pad controller 23 can beconfigured to control the connections between the cascaded buildingblocks. For example, the pad controller 23 can be configured to bypassthe second building block 74, thus selectively omitting the secondbuilding block 74 from the cascade. In one embodiment, a first buildingblock is formed below the pad and additional building blocks are formedin the vicinity of the pad. After completing both front-end and back-endfabrication, particular building blocks can be included in a cascadewith the first building block using the pad controller 23. For example,the pad controller 23 can be configured to include or exclude particularbuilding blocks, thereby configuring the pad circuit 22 to have thetrigger voltage V_(TRIGGER) and holding voltage V_(HOLDING) desired fora particular application. In one embodiment, each pad circuit 22 can beindividually controlled by the pad controller 23 to achieve the desiredcascade. In alternative embodiments, groupings of pads can becollectively configured by the pad controller 23. This can be desirable,for example, when a particular group of pads, such as VH1 and VL1 ofFIG. 2, may have similar performance and reliability requirements.

In one embodiment, the pad controller 23 is configured to use metal orpoly fuses to control the operation of an ESD tolerant switch. Theswitch can be configured to bypass the operation of particular buildingblocks in the pad circuit 22. In an alternate embodiment, the padcontroller 23 can include a multitude of fuse-controlled filaments thatcan be independently biased to configure each pad circuit 22 percombinations of building block types, such as the building block typeswhich will be described later with reference to FIGS. 5A-5C.

Although FIGS. 4A and 4B were described in the context of Type A andType B building blocks, additional building block types can be used. Forexample, a Type C building block can have a holding voltage V_(H) _(—)_(C) and a trigger voltage V_(T) _(—) _(C) that are different from theholding voltages and the trigger voltages, respectively, of the firstand second types of building blocks. The pad circuit 22 can combine inumber of Type A building blocks, j number of Type B building blocks,and k number of Type C building blocks such that the pad circuit 22 hasa trigger voltage V_(TRIGGER) roughly equal to about i*V_(T) _(—)_(A)+j*V_(T) _(—) _(B)+k*V_(T) _(—) _(C), and a holding voltageV_(HOLDING) roughly equal to about i*V_(H) _(—) _(A)+j*V_(H) _(—) _(B)k*V_(H) _(—) _(C). The inclusion of additional building block types canincrease the multitude of configurations of the cascade at the expenseof an increase in design complexity. Furthermore, the number of buildingblocks in the cascade can also be increased to provide additionalconfigurations, provided that each building block remains properlybiased at the increased trigger and holding voltages. For example, in anelectrically isolated clamp embodiment in which a deep n-well layerprovides electrical isolation between building blocks, the number ofbuilding blocks can be limited by the voltage level provided to the deepn-well to maintain electrical isolation.

FIGS. 5A-5C illustrate the circuits of a family of building block types,one or more of which can be employed as a building block type in the padcircuits of FIGS. 4A and 4B.

FIG. 5A is a circuit diagram illustrating a pad circuit building block(for example, the Type A building block described above in connectionwith FIGS. 4A and 4B) in accordance with one embodiment. The Type Abuilding block 91 includes a resistor 101 and a NPN bipolar transistor100 having an emitter, a base, and a collector. The resistor 101includes a first end electrically connected to the base of thetransistor 100, and a second end electrically connected to the emitterof the transistor 100. The resistor 101 can have, for example, aresistance between about 5Ω and about 55Ω. The collector of thetransistor 100 can be electrically connected to another building blockor to a pad 42. The emitter of the transistor 100 can be electricallyconnected to another building block or to a node 82.

FIG. 5B is a circuit diagram illustrating a pad circuit building block(for example, the Type B building block described above in connectionwith FIGS. 4A and 4B) in accordance with another embodiment. The Type Bbuilding block 92 includes a PNP bipolar transistor 102, an NPN bipolartransistor 103, a first resistor 104 and a second resistor 105. The PNPtransistor 102 and the NPN transistor 103 each include an emitter, abase, and a collector. The first resistor 104 includes a first endelectrically connected to the emitter of the PNP transistor 102, and asecond end electrically connected to the base of the PNP transistor 102and to the collector of the NPN transistor 103. The first resistor 104can have, for example, a resistance between about 5Ω and about 35Ω. Thesecond resistor 105 includes a first end electrically connected to thecollector of the PNP transistor 102 and to the base of the NPNtransistor 103, and a second end electrically connected to the emitterof the NPN transistor 103. The second resistor 105 can have, forexample, a resistance between about 50Ω and about 250Ω. The emitter ofthe PNP transistor 102 can be electrically connected to another buildingblock or to a pad 42. The emitter of the NPN transistor 103 can beconnected to another building block or to a node 82.

As skilled artisans will appreciate, the PNP transistor 102 and NPNtransistor 103 are configured to be in feedback. At a certain level ofthe collector current of the PNP transistor 102, the feedback betweenthe PNP transistor 102 and the NPN transistor 103 can be regenerativeand can cause the Type B building block 92 to enter a low-impedancestate.

FIG. 5C is a circuit diagram illustrating a pad circuit building block(for example, the Type C building block described above in connectionwith FIGS. 4A-4B) in accordance with yet another embodiment. The Type Cbuilding block 93 includes a resistor 107 and a PNP bipolar transistor106 having an emitter, a base, and a collector. A first end of theresistor 107 is electrically connected to the emitter of the transistor106, and a second end is electrically connected to the base of thetransistor 106. The resistor 107 can have, for example, a resistancebetween about 11Ω and about 85Ω. The emitter of the transistor 106 canbe electrically connected to another building block or to a pad 42. Thecollector of the transistor 106 can be connected to another buildingblock or to a node 82.

With reference to FIGS. 5A-5C, the trigger and holding voltages of theType A, Type B, and Type C building blocks can be selected so as to aidin configuring the pad circuit 22 to have a trigger voltage V_(TRIGGER)and a holding voltage V_(HOLDING) desired for a particular electronicsystem or application. For example, the trigger voltage of the Type Abuilding block V_(T) _(—) _(A) and the trigger voltage of the Type Bbuilding block V_(T) _(—) _(B) can be based on the collector-emitterbreakdown voltage of the NPN transistor 100 and the NPN transistor 103,respectively. Additionally, the positive feedback between the NPNtransistor 103 and the PNP transistor 102 in Type B Building block 92can make the holding voltage V_(H) _(—) _(B) of the Type B buildingblock 92 less than the holding voltage V_(H) _(—) _(A) of the Type Abuilding block 91. Furthermore, the Type C building block can have aholding voltage V_(H) _(—) _(C) greater than either the holding voltageV_(H) _(—) _(A) or V_(H) _(—) _(B), and can have a trigger voltage V_(T)_(—) _(C) based on the collector-emitter breakdown voltage of the PNPtransistor 106.

In one embodiment, the Type A building block 91 and the Type B buildingblock 92 are configured to have about the same trigger voltage, V_(T)_(—) _(A)=V_(T) _(—) _(B)=V_(T). Additionally, the positive feedbackbetween the NPN transistor 103 and the PNP transistor 102 is employed toselectively decrease the holding voltage V_(H) _(—) _(B) of the Type Bbuilding block 92 relative to the holding voltage V_(H) _(—) _(A) of theType A building block. Thus, in some embodiments, i number of Type Abuilding blocks and j number of Type B building blocks can be combinedin a cascade configuration to produce a pad circuit 22 having a triggervoltage V_(TRIGGER) roughly equal to about (i+j)*V_(T), and a holdingvoltage V_(HOLDING) roughly equal to about i*V_(H) _(—) _(A) j*V_(H)_(—) _(B), where V_(H) _(—) _(B) is selected to be less than V_(H) _(—)_(A). This permits configurations having the same number of buildingblocks in the cascade to have about the same trigger voltageV_(TRIGGER). Additionally, the type of building blocks in the cascadecan be selected to achieve the desired holding voltage V_(HOLDING) ofthe pad circuit 22.

Skilled artisans will appreciate that the desired trigger voltage andholding voltage of each building block type can be achieved by properselection of a variety of parameters, including, for example, thegeometries of the transistors, the common-emitter gain or “β” of thetransistors, and by selecting the resistance of the resistors.

Bipolar Transistor Structures for Pad Circuits

FIGS. 6A-6C illustrate cross sections of various transistor structures.As will be described below, FIGS. 6B and 6C illustrate cross sections oftransistor structures according to embodiments of the invention. Thesetransistors can be used in pad circuit building blocks, even inprocesses lacking dedicated bipolar transistor masks.

FIG. 6A illustrates a cross section of a conventional NMOS transistorhaving a lightly doped drain (LDD) structure. The LDD NMOS transistor120 is formed on a substrate 121 and includes an n+ drain region 122, ann+ source region 123, a gate 125, gate oxide 127, a lightly doped (n−)drain extension region 128, a lightly doped source extension region 129,and sidewall spacers 130.

The n+ drain region 122 can be more heavily doped than the n− drainextension region 128. The difference in doping can reduce the electricfields near the drain region, thereby improving the speed andreliability of the transistor 120 while lowering gate-drain capacitanceand minimizing the injection of hot electrons into the gate 125.Likewise, the n+ source region 123 can be more heavily doped than the n−source extension region 129 and provide similar improvements to thetransistor 120.

In a conventional LDD process, the gate electrode 125 is used as a maskfor n− LDD implantation used to form the drain and source extensionregions 128, 129. Thereafter, sidewall spacers 130 can be provided andemployed as a mask for n+ implantation used to form the drain region 122and the source region 123.

FIG. 6B illustrates a cross section of a parasitic NPN bipolartransistor in accordance with one embodiment. The illustrated parasiticNPN bipolar transistor 140 includes an emitter 141, a base 142 formed ofa p-well, a collector 143, a plate 145, an oxide layer 147, an isolationlayer 151, and sidewall spacers 150. The emitter 141, the collector 143,the plate 145, and the oxide layer 147 have structures similar to thoseof the drain region 122, the source region 123, the gate 125, and theoxide layer 127, respectively, of the conventional NMOS transistor 120of FIG. 6A. In contrast to the LDD NMOS transistor 120 shown in FIG. 6A,the illustrated bipolar transistor 140 does not have structures similarto those of the source and drain extension regions of the NMOStransistor 120.

Removal of the source and drain extension regions can result intransistor conduction being dominated by a bipolar component, ratherthan by a FET component. In particular, when a voltage is applied to theplate 145, the inversion layer may not extend from the emitter 141 tothe collector 143, and thus the FET component of the current can beweak. Thus, during an overvoltage condition, the parasitic NPN bipolartransistor 140 can serve as the primary conduction path, and theparasitic NPN bipolar transistor 140 can function similarly to atraditional bipolar transistor.

The resulting structure can have lower leakage than a conventional NMOSstructure and withstand relatively large voltages without breakdown.Further, the resulting structure can be sized so as to employ theparasitic bipolar structure for transient signal protection withoutdrawbacks, such as reduced reliability, typically encountered in highperformance analog applications when degrading the standard MOS devicecharacteristics. Since the parasitic NPN bipolar transistor 140 can beformed using a process used to create a conventional LDD MOS transistor,such as the NMOS transistor 120 of FIG. 6A, both the parasitic NPNbipolar transistor 140 and the LDD NMOS transistor 120 can be fabricatedsimultaneously on a common substrate.

The parasitic bipolar transistor 140 can have desirable properties forESD protection and can be used in building blocks described above inconnection with FIGS. 5A-5B. The use of the parasitic NPN bipolartransistor 140 can be desirable, for example, in a process whichincludes conventional LDD MOS transistors, but which lacks a dedicatedbipolar process. In one embodiment, a single additional mask can beadded during fabrication of transistors to determine which transistorstructures receive the LDD implant and which do not.

The sidewall spacers 150 can be formed using, for example, an oxide,such as SiO₂, or a nitride. However, other sidewall spacer materials canbe utilized in certain manufacturing processes. A distance x₁ betweenthe emitter 141 and the plate 145 can be selected to be, for example, ina range of about 0.1 μm to 2.0 μm. A distance x₂ between the collector143 and the plate 145 can be selected to be, for example, in a range ofabout 0.1 μm to 2.0 μm.

The plate 145 can be formed from a variety of materials, including, forexample, doped or undoped polysilicon. Although the plate 145 isillustrated as a single layer, the plate 145 can include multiplelayers, such as, for example, layers of polysilicon and silicide. In oneembodiment, the plate 145 can have a plate length x₃ selected to be in arange of about 0.25 μm to about 0.6 μm, for example, about 0.5 μm.However, skilled artisans will appreciate that the length of the plate145 can vary depending on the particular process and application. Theplate 145 can be formed over the oxide layer 147, which can correspondto, for example, any oxide layer dielectric known in the art or anyoxide layer dielectric later discovered, including high-k oxide layers.

The emitter 141 and the collector 143 of the bipolar transistor 140 canbe formed using a variety of materials, including for example, anyn-type doping material. The spacing between the emitter 141 and thecollector 143 can correspond to the sum of the distance x1, the distancex2, and the plate length x3. In one embodiment, the spacing between theemitter 141 and collector 143 is selected to be in the range of about0.45 μm to about 4.6 μm. The doping between the emitter and thecollector, both beneath the sidewall spacers 151 and the plate canconsist essentially of n-type, which can result in transistor conductionbeing dominated by a bipolar component, rather than by a FET component.Thus, when a voltage is applied to the plate 145, the inversion layermay not extend from the emitter 141 to the collector 143, and thus theFET component of the current can be weak. Accordingly, during anovervoltage condition, the parasitic NPN bipolar transistor 140 canserve as the primary conduction path, and the parasitic NPN bipolartransistor 140 can function similarly to a traditional bipolartransistor.

The base 142 can be electrically isolated from the substrate 144 using awide variety of techniques. In the illustrated embodiment, the isolationlayer 151 is a deep n-well layer provided to electrically isolate thebase 142 from the substrate 144. Persons of ordinary skill in the artwill appreciate that a variety of techniques to provide electricalisolation are well known in the art and can be used in accordance withthe teachings herein. For example, the isolation layer 151 can be ann-type buried layer or an isolation layer of a silicon-on-insulator(SOI) technology. The parasitic bipolar transistor 140 can undergo backend processing to form, for example, contacts and metallization. Skilledartisans will appreciate that various processes can be used for suchback end processing.

FIG. 6C is a cross section of a PNP bipolar transistor 160 in accordancewith one embodiment. The illustrated PNP bipolar transistor 160 includesan emitter 161, a base 162 formed of an n-well, a collector 163, a plate165, an oxide layer 167, and sidewall spacers 170. The PNP bipolartransistor 160 can be formed in a manner similar to that of the NPNbipolar transistor 140 by selecting impurities with opposite polarity tothat described above.

The parasitic NPN bipolar transistor 140 and the parasitic PNP bipolartransistor 160 can be formed by omitting the implantation of the LDDlayer in a conventional MOS process. As will be described in detailbelow, the NPN bipolar transistor 140 and the PNP bipolar transistor 160can be used in the building blocks of FIGS. 5A-5C, thereby permittingthe fabrication of a family of pad circuit building blocks even with aprocess lacking dedicated bipolar masks. The building blocks can becascaded to achieve the desired holding and trigger voltages for a padcircuit, such as the pad circuit 22 of FIGS. 4A and 4B.

Alternative Embodiments of IC Pad Circuits

FIGS. 7A-8B represent building block types, one or more of which can beemployed as a building block type in the pad circuits of FIGS. 4A and4B.

FIG. 7A is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment. The illustrated Type A′building block 201 can be connected in a cascade between a pad 42 and anode 82, and includes a first resistor 203, a second resistor 205, adiode 204, and a NPN bipolar transistor 202 having an emitter, a base, acollector, and a plate. The NPN bipolar transistor 202 can have thestructure of the NPN bipolar transistor 140 of FIG. 6B.

The diode 204 includes an anode electrically connected to the node 82,and a cathode electrically connected to the collector of the NPN bipolartransistor 202 at a node N₁. The node N₁ can be electrically connectedto another building block in a cascade, such as the cascade of FIG. 4A,or to the pad 42. The first resistor 203 includes a first endelectrically connected to the base of the NPN bipolar transistor 202,and a second end electrically connected to the emitter of the NPNbipolar transistor 202 and to a first end of the second resistor 205 ata node N₂. The first resistor 203 can have, for example, a resistancebetween about 5Ω and about 55Ω. In one embodiment, described below withreference to FIG. 7B, the first resistor 203 is implemented using amulti-finger array to achieve the target resistance, such as an array ofsix fingers each having a resistance selected from the range of about30Ω and about 320Ω. The node N₂ can be electrically connected to anotherbuilding block in a cascade or to the node 82. The second resistor 205includes a second end electrically connected to the plate of the NPNbipolar transistor 202. The second resistor 205 can have, for example, aresistance between about 50Ω and about 50 kΩ.

As was described before with reference to FIGS. 4A and 4B, the padcircuit 22 can be employed in, for example, any of the pad circuits 22a-22 p shown in FIG. 2, and the pad 42 can be any of the pads 42 a-42 p,including, for example, low-impedance output pads, high-impedance inputpads, and low-impedance power pads. The node 82 can be, for example, alow impedance node or pad of the power management IC 20 configured tohandle a relatively large shunted current. A transient signal event canbe received at the pad 42. If the transient signal event has a voltagewhich is negative with respect to the node 82, the diode 204 can providecurrent which can aid in protecting the power management IC 20.

If the transient signal event has a voltage that is positive withrespect to the node 82, the NPN bipolar transistor 202 can aid inproviding transient signal protection. The trigger voltage of the TypeA′ building block V_(T) _(—) _(A′) can be based on the collector-emitterbreakdown voltage of the NPN bipolar transistor 202. Additionally, theplate and the collector of the NPN bipolar transistor 202 can functionto form a capacitor, which can enhance how the NPN bipolar transistor202 performs when a transient signal event having a positive voltage isreceived by increasing the displacement current, as will be describedbelow.

If the transient signal event received on pad 42 causes the node N₁ tohave a rate of change dV_(N1)/dt and the capacitance between the plateand the collector of the NPN bipolar transistor 202 has a value of C₂₀₂,a displacement current can be injected by the capacitor equal to aboutC₂₀₂*dV_(N1)/dt. A portion of this current can be injected in the baseof the NPN bipolar transistor 202, which can increase the speed at whichthe Type A′ building block 201 provides transient signal protection. Asdescribed above, a transient signal event can be associated with fastrise and fall times (for example, from about 0.1 ns to about 1.0 ms)relative to the range of normal signal operating conditions. Thus, theNPN bipolar transistor 202 can be configured to have a trigger voltagewhich decreases in response to rates of voltage change associated withthe very high frequency conditions of a transient signal event. Duringnormal operation, the absence of the lightly doped drain (LDD) can makethe leakage of the NPN bipolar transistor 202 relatively low, even overa relatively wide range of temperatures, for example, between about −40°C. and about 140° C.

FIG. 7B illustrates an annotated cross section of one implementation ofthe pad circuit building block of FIG. 7A. The illustrated Type A′building block 201 includes a substrate 221, emitters 211 a-211 f, base212, collectors 213 a-213 e, plates 215 a-215 j, base contacts 217 a,217 b, n-wells 218 a, 218 b, deep n-well 219, and substrate contacts 220a, 220 b. The cross section has been annotated to illustrate examples ofcircuit devices formed, such as parasitic NPN bipolar transistors 202a-202 j, resistors 203 a, 203 b, and diodes 204 a, 204 b. The diagram isalso annotated to show the second resistor 205, which can be formedusing, for example, n-diffusion or poly (not shown in this Figure). TheType A′ building block 201 can undergo back end processing to formcontacts and metallization. These details have been omitted from FIG. 7Bfor clarity.

The diodes 204 a, 204 b can be formed from the substrate 221 and n-wells218 a, 218 b. For example, the diode 204 a has an anode formed from thesubstrate 221 and a cathode formed from the n-well 218 a. Similarly, thediode 204 b has an anode formed from the substrate 221 and a cathodeformed from the n-well 218 b.

The NPN bipolar transistors 202 a-202 j can be formed from emitters 211a-211 f, collectors 213 a-213 e, plates 215 a-215 j, and base 212. Forexample, the NPN bipolar transistor 202 a can be formed from the emitter211 a, the plate 215 a, the collector 213 a, and the base 212. The NPNbipolar transistors 202 b-202 j can be formed in a similar manner fromemitters 211 b-211 f, collectors 213 a-213 e, plates 215 b-215 j, andbase 212. Additional details of the NPN bipolar transistors 202 a-202 jcan be as described above with reference to FIG. 6B.

The base 212 can be electrically isolated from the substrate 221 usingn-wells 218 a, 218 b and deep n-well 219. The n-wells 218 a, 218 b anddeep n-well 219 can also provide electrically isolation of the buildingblock from other building blocks. The n-well contacts 222 a, 222 b canform a guard ring around the Type A′ building block 201. The n-wellcontacts 222 a, 222 b can be contacted to a metal layer above by usingmultiple rows of contacts, thereby permitting the guard ring to beconnected to the collectors 213 a-213 e through metal. The guard ringcan eliminate the formation of unintended parasitic paths between thepad circuit and surrounding semiconductor components when integratedon-chip. Additionally, the substrate contacts 220 a, 220 b can form asubstrate ring which can aid in protecting the Type A′ building block201 from latch-up.

The resistors 203 a, 203 b can be formed from the resistance between thebases of NPN bipolar transistors 202 a-202 j and the base contacts 217a, 217 b. The resistance along the paths between the bases of the NPNbipolar transistors 202 a-202 j and the base contacts 217 a, 217 b canbe modeled by the resistors 203 a, 203 b.

Persons of ordinary skill in the art will appreciate that thecross-section shown in FIG. 7B can result in the formation of thecircuit shown in FIG. 7A. For example, each of the emitters of the NPNbipolar transistors 202 a-202 j can be electrically connected togetherto form a common emitter. Likewise, each of the collectors, plates, andbases of the NPN bipolar transistors 202 a-202 j can be electricallyconnected together to form a common collector, a common plate, and acommon base, respectively. Thus, each of the NPN bipolar transistors 202a-202 j can be legs of the NPN bipolar transistor 202. Additionally, thediodes 204 a, 204 b can be represented by the diode 204, and theresistors 203 a, 203 b can be represented by the first resistor 203. Thesecond resistor 205 can be formed using, for example, n-diffusion orpoly (not shown in this Figure). Thus, FIG. 7B illustrates a crosssection of an implementation of the pad circuit building block of FIG.7A. Skilled artisans will appreciate that numerous layoutimplementations of the Type A′ building block 201 are possible.

As described earlier with reference to FIG. 7A, the capacitance betweenthe plate and the collector of the NPN bipolar transistor 202 can resultin a current which can be injected in the base of the NPN bipolartransistor 202. This can increase the speed at which the Type A′building block 201 provides transient signal protection. The secondresistor 205 can have a resistance selected to provide injection intothe base of the NPN bipolar transistors at a frequency associated with atransient signal event. In one embodiment, the second resistor 205 canhave a resistance in the range of about 200Ω to 50 kΩs.

Each of the NPN bipolar transistors 202 a-202 j can be legs of the NPNbipolar transistor 202 as described above. In one embodiment, each ofthe NPN bipolar transistors has a plate width (for example, the width ofthe plate 145 in a direction orthogonal to the plate length x₃ of FIG.6B) between about 30 μm and 100 μm, so that the total plate width (thesum of the plates widths of all legs) is in the range of about 300 μm to1,000 μm. In one embodiment, the plate length of each NPN bipolartransistors (for example, x₃ in FIG. 6B) is selected to be between about0.25 μm and about 0.6 μm, for example, about 0.5 μm. Although the crosssection shown in FIG. 7B illustrates the NPN bipolar transistor 202 ashaving ten legs, skilled artisans will appreciate that more or fewerlegs can be selected depending on, for example, the desired dimensionsof the pad circuit and the desired total plate width. In one embodimentdescribed with reference to FIGS. 17A-17H, the number and width of thelegs are selected so that the implementation of the Type A′ buildingblock 201 can fit under a bonding pad.

FIG. 8A is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment. The illustrated Type B′building block 231 can be connected in a cascade between the pad 42 andthe node 82, and includes a PNP transistor 232, a NPN bipolar transistor233, a first resistor 234, a second resistor 235, a third resistor 236,and a diode 237. The PNP transistor 232 includes an emitter, a base, anda collector. The NPN bipolar transistor 233 includes an emitter, a base,a collector and a plate, and can have a structure similar to that of theNPN bipolar transistor 140 of FIG. 6B.

The diode 237 includes an anode electrically connected to the node 82,and a cathode electrically connected to a first end of the firstresistor 234 and to the emitter of the PNP transistor 232 at a node N₃.The node N₃ can be electrically connected to another building block in acascade, such as the cascade of FIG. 4A, or to the pad 42. The firstresistor 234 also includes a second end electrically connected to thebase of the PNP transistor 232 and to the collector of the NPN bipolartransistor 233. The first resistor 234 can have, for example, aresistance between about 5Ω and about 35Ω. In one embodiment, describedbelow with reference to FIG. 8B, the first resistor 234 is implementedusing a multi-finger array to achieve the target resistance, such as anarray of two fingers each having a resistance selected from the range ofabout 10Ω and about 70Ω. The second resistor 235 includes a first endelectrically connected to the collector of the PNP transistor 232 and tothe base of the NPN bipolar transistor 233, and a second endelectrically connected to the emitter of the NPN bipolar transistor 233and to a first end of the third resistor 236 at a node N₄. The secondresistor 235 can have, for example, a resistance between about 50Ω andabout 250Ω. In one embodiment, described below with reference to FIG.8B, the second resistor 235 is implemented using a multi-finger array toachieve the target resistance, such as an array of two fingers eachhaving a resistance selected from the range of about 100Ω and about500Ω. The node N₄ can be electrically connected to another buildingblock in a cascade or to the node 82. The third resistor 236 includes asecond end electrically connected to the plate of the NPN bipolartransistor 233. The third resistor 236 can have, for example, aresistance between about 200Ω and about 50 kΩ.

As was described before with reference to FIGS. 4A and 4B, the padcircuit 22 can be, for example, any of the pad circuits 22 a-22 p shownin FIG. 2, and the pad 42 can be any of the pads 42 a-42 p. The node 82can be, for example, a low impedance node or pad of the power managementIC 20 configured to handle a relatively large shunted current. Atransient signal event can be received at the pad 42. If the transientsignal event has a voltage that is negative with respect to the node 82,the diode 237 can provide current which can aid in protecting the powermanagement IC 20.

If the transient signal event has a voltage which is positive withrespect to the node 82, the PNP transistor 232 and the NPN bipolartransistor 233 can aid in providing transient signal protection. Thetrigger voltage of the Type B′ building block V_(T) _(—) _(B′) can bebased on the collector-emitter breakdown voltage of the NPN bipolartransistor 233. Additionally, the positive feedback between the NPNbipolar transistor 233 and the PNP transistor 232 can make the holdingvoltage V_(T) _(—) _(B′) of the Type B′ building block 231 less than theholding voltage V_(H) _(—) _(A′) of the Type A′ building block 201 ofFIG. 7A.

The plate and the collector of the NPN bipolar transistor 233 canfunction to form a capacitor which can enhance the performance of theNPN bipolar transistor 233 when a transient signal event having apositive voltage is received, as was described earlier. For example, aportion of this current can be injected in the base of the NPN bipolartransistor 233 through capacitive coupling, which can aid the speed atwhich the Type B′ building block 231 provides transient signalprotection. Thus, the NPN bipolar transistor 233 can be configured tohave a trigger voltage which is lower at rates of voltage changeassociated with the very high frequency conditions of a transient signalevent. During normal operation, the absence of the lightly doped drain(LDD) can make the leakage of the NPN bipolar transistor 233 low, evenat relatively high temperatures.

FIG. 8B is an annotated cross section of one implementation of the padcircuit building block of FIG. 8A. The illustrated Type B′ buildingblock 231 includes NPN emitters 241 a, 241 b, NPN bases 242 a, 242 b,NPN collector contacts 243 a, 243 b, plates 245 a, 245 b, NPN basecontacts 247 a, 247 b, PNP base 258, PNP base contacts 257 a, 257 b,n-wells 248 a, 248 b, deep n-well 249, and substrate contacts 250 a, 250b. As illustrated, the NPN collector contacts 243 a, 243 b are eachformed partially in a p-well and partially in an n-well. For example,the NPN collector contact 243 a is partially formed in the NPN base 242a, and partially formed in the PNP base 258, and the NPN collectorcontact 243 b is partially formed in the NPN base 242 b and partiallyformed in the PNP base 258. The cross section has been annotated to showcertain circuit components formed from the layout, including NPN bipolartransistors 233 a, 233 b, PNP transistors 232 a, 232 b, p-well resistors235 a, 235 b, n-well resistors 234 a, 234 b, and diodes 237 a, 237 b.The diagram is also annotated to show the third resistor 236, which canbe formed using, for example, n-diffusion (not shown in this Figure).The Type B′ building block 231 can undergo back end processing to formcontacts and metallization. These details have been omitted from FIG. 8Bfor clarity.

The diodes 237 a, 237 b can be formed from substrate 251 and n-wells 248a, 248 b. For example, the diode 237 a has an anode formed from thesubstrate 251 and a cathode formed from the n-well 248 a. The diode 237b has an anode formed from the substrate 251 and a cathode formed fromthe n-well 248 b.

The NPN bipolar transistors 233 a, 233 b can be formed from NPN emitters241 a, 241 b, PNP base 258, NPN collector contacts 243 a, 243 b, plates245 a, 245 b, and NPN bases 242 a, 242 b. For example, the NPN bipolartransistor 233 a can be formed from the NPN emitter 241 a, the plate 245a, the PNP base 258, the NPN collector contact 243 a, and the NPN base242 a. Likewise, the NPN bipolar transistor 233 b can be formed from theNPN emitter 241 b, the plate 245 b, the PNP base 258, the NPN collectorcontact 243 b, and the NPN base 242 b. Although the NPN bipolartransistors 233 a, 233 b are connected to NPN collector contacts 243 a,243 b, in the illustrated embodiment, the contacts 243 a, 243 b are notconnected to metal layers, and thus the PNP base 258 can also serve asthe collectors for NPN bipolar transistors 233 a, 233 b. Additionaldetails of the NPN bipolar transistors 233 a, 233 b can be found abovewith reference to FIG. 6B.

The NPN bases 242 a, 242 b can be electrically isolated using n-wells248 a, 248 b, n-well of the PNP base 258, and deep n-well 249. Then-well contacts 252 a, 252 b can form part of a guard ring around theType B′ building block 231. The substrate contacts 250 a, 250 b can forma portion of a substrate ring which can aid in protecting the Type B′building block 231 from latch-up.

The p-well resistors 235 a, 235 b can be formed from the resistancebetween the bases of NPN bipolar transistors 233 a, 233 b and the basecontacts 247 a, 247 b. Skilled artisans will appreciate that the p-wellsof the bases 242 a, 242 b can have a resistivity along the electricalpath between the bases of NPN bipolar transistors 233 a, 233 b and thebase contacts 247 a, 247 b, which can be modeled by p-well resistors 235a, 235 b.

The PNP transistors 232 a, 232 b can be formed from PNP emitters 254 a,254 b, PNP base 258, and the NPN bases 242 a, 242 b. For example, thePNP transistor 232 a can have an emitter formed from the PNP emitter 254a, a base formed from the PNP base 258, and a collector formed from theNPN base 242 a. Likewise, the PNP transistor 232 b can have an emitterformed from the PNP emitter 254 b, a base formed from the PNP base 258,and a collector formed from the NPN base 242 b.

The n-well resistors 234 a, 234 b can be formed from the resistancebetween the bases of PNP transistors 232 a, 232 b and the PNP basecontacts 257 a, 257 b. Skilled artisans will appreciate that the n-wellof the PNP base 258 can have a resistivity along the electrical pathbetween the bases of PNP transistors 232 a, 232 b and the PNP basecontacts 257 a, 257 b, which can be modeled by n-well resistors 234 a,234 b.

Persons of ordinary skill in the art will appreciate that thecross-section shown in FIG. 8B can result in the formation of thecircuit shown in FIG. 8A. For example, each of the NPN bipolartransistors 233 a, 233 b can be legs of the NPN bipolar transistor 233.Likewise, each of the PNP transistors 232 a, 232 b can be legs of thePNP transistor 232. Additionally, the diodes 237 a, 237 b can form thediode 237, the n-well resistors 234 a, 234 b can form the first resistor234, and the p-well resistors 235 a, 235 b can form the second resistor235. The third resistor 236 can be formed using, for example,n-diffusion or poly (not shown in this Figure). Thus, FIG. 8B is a crosssection of one implementation of the of the pad circuit building blockof FIG. 8A. Skilled artisans will appreciate that numerous variations ofthe Type B′ building block 201 are possible.

As was described above with reference to FIG. 8A, when a transientsignal is present, the capacitance between the plate and the collectorof the NPN bipolar transistor 233 can result in a current being injectedin the base of the NPN bipolar transistor 233. This can aid the speed atwhich the Type B′ building block 231 provides transient signalprotection. The third resistor 236 can have a resistance selected toprovide injection into the base of the NPN bipolar transistor 233 at afrequency associated with a particular transient signal event. In oneembodiment, the third resistor 236 has a resistance selected in therange of about 200Ω to 50 kΩs.

Each of the NPN bipolar transistors 233 a, 233 b can be legs of the NPNbipolar transistor 233. In one embodiment, each NPN bipolar transistor233 a, 233 b has a plate width typically selected between about 30 μmand 50 μm, so that the total plate width of the NPN bipolar transistor233 is in the range of about 60 μm to 100 μm. The length of each NPNbipolar transistor 233 a, 233 b can have a length selected between, forexample, about 0.25 μm and 0.6 μm, for example, about 0.5 μm. Althoughthe cross section in FIG. 8B shows the NPN bipolar transistor 233 ashaving two legs, skilled artisans will appreciate that additional orfewer legs can be selected depending on a variety of factors, includingthe desired pad circuit dimensions and the desired total plate width. Inone embodiment described with reference to FIGS. 18A-18B, the number andwidth of the legs is selected so that two instantiations of the Type B′building block 231 can fit under a bonding pad.

The PNP transistors 232 a, 232 b can be legs of the PNP transistor 232.Although the cross section illustrated in FIG. 8B shows the PNPtransistor 232 as having two legs, skilled artisans will appreciate thatadditional or fewer legs can be selected depending on a variety offactors such as the manufacturing process and application.

With reference to FIGS. 4A, 4B, 7A, and 8A, the trigger voltages V_(T)_(—) _(A′), V_(T) _(—) _(B′) and the holding voltages V_(H) _(—) _(A′),V_(H) _(—) _(B′) of the Type A′ and Type B′ building blocks can beselected so that the pad circuit 22 has a trigger voltage V_(TRIGGER)and a holding voltage V_(HOLDING) desired for a particular electronicsystem or application. For example, i number of Type A′ building blocksand j number of Type B′ building blocks can be cascaded so that the padcircuit 22 has a trigger voltage V_(TRIGGER) roughly equal to abouti*V_(T) _(—) _(A′)+j*V_(T) _(—) _(B′), and a holding voltage V_(HOLDING)roughly equal to about i*V_(H) _(—) _(A′)+j/*V_(H) _(—) _(B′). Byselecting the Type and number of building blocks employed, and/or byselecting the value of V_(H) _(—) _(A′), V_(H) _(—) _(B′), V_(T) _(—)_(A′) and V_(T) _(—) _(B′) during design of the building blocks, ascalable family of pad circuits can be created which can be adapted fora multitude of electronic systems and applications. The design costassociated with designing the pad circuits can be reduced as comparedto, for example, an approach in which different diode, bipolar, siliconcontrolled rectifier and MOS devices are employed to achieve thereliability and performance requirements needed for each pad circuit.The desired trigger voltage and holding voltage of each building blocktype can be achieved by proper selection of a variety of parameters,including, for example, the geometries of the transistors, thecommon-emitter gain or “β” of the transistors, and by selecting theresistance of the resistors.

In one embodiment, the Type A′ building block 201 and the Type B′building block 231 are configured to have about the same triggervoltage, V_(T) _(—) _(A′)=V_(T) _(—) _(B′)=V_(T′). Additionally, thepositive feedback between the NPN bipolar transistor 233 and the PNPtransistor 232 is employed to selectively decrease the holding voltageV_(H) _(—) _(B′) of the Type B′ building block 231 relative to theholding voltage V_(H) _(—) _(A′) of the Type A′ building block 201.Thus, i number of Type A′ building blocks and j number of Type B′building blocks can be combined in a cascade configuration to produce apad circuit 22 having a trigger voltage V_(TRIGGER) roughly equal toabout (i+j)*V_(T′), and a holding voltage V_(HOLDING) roughly equal toabout i*V_(H) _(—) _(A′)+j*V_(H) _(—) _(B′), where V_(H) _(—) _(B′) isselected to be less than V_(H) _(—) _(A′). This permits configurationshaving the same number of building blocks in the cascade to have aboutthe same trigger voltage V_(TRIGGER). Additionally, the type of buildingblocks in the cascade can be selected to achieve the desired holdingvoltage V_(HOLDING) of the pad circuit 22.

FIGS. 9A-14B illustrate various other embodiments in a family ofcascaded building blocks using Type A′ building block 201 and Type B′building block 231. Although FIGS. 9A-14B are described in the contextof Type A′ and Type B′ building blocks 201, 231 of FIGS. 7A and 8A,skilled artisans will appreciate that similar configurations can becreated using the Type A and Type B building blocks 91, 92 of FIGS. 5Aand 5B.

As was described earlier with reference to Table 1 and FIGS. 3A and 3B,there is a need for pad circuits which can be configured to meet theperformance and design parameters required for a particular application.For example, various pads of the power management IC 20 can havedifferent reliability and performance parameters, as shown in Table 1.FIGS. 9A-14B illustrate various cascade configurations of Type A′ andType B′ building blocks 201, 231, which can be employed to meetdifferent reliability and performance parameters, as will be describedbelow. In one embodiment, the type and number of building blocks areselected during design for a particular application. In anotherembodiment, a multitude of building blocks are placed in the vicinity ofthe pad during front end fabrication, and the desired configuration isselected by changing metal layers and via connections during back endprocessing. In yet another embodiment, a multitude of building blocksare placed in the vicinity of the bonding pad, and the type and numberof the building blocks are selected using the pad controller 23 afterfabrication, as was described earlier.

FIG. 9A is a schematic block diagram of a pad circuit according to afirst embodiment. The illustrated pad circuit 281 includes two Type A′building blocks 201 connected in a cascade between the pad 42 and thenode 82. The Type A′ building block 201 can be configured to have atrigger voltage V_(T) _(—) _(A′) equal to about the trigger voltageV_(T) _(—) _(B′) of the Type B′ building block 231 of FIG. 8A. However,the holding voltage V_(H) _(—) _(A′) of the Type A′ building block 201can be configured to be greater than the holding voltage V_(H) _(—)_(B′) of the Type B′ building block 231. Thus, the pad circuit 281 canbe employed, for example, in an input pad having a moderate operatingvoltage and requiring a relatively high holding voltage. For example, ifV_(T) _(—) _(A′) is equal to about 9 V and V_(H) _(—) _(A′) is equal toabout 5 V, the pad circuit 281 can have a trigger voltage of about 18 Vand a holding voltage of about 10 V. Thus, the pad circuit 281 can havea holding voltage and trigger voltage appropriate for the pad VH1 inTable 1.

FIG. 9B is a circuit diagram of the pad circuit of FIG. 9A. Theillustrated pad circuit 281 includes two Type A′ building blocksconnected in a cascade configuration between the pad 42 and the node 82.Each Type A′ building block 201 includes a first resistor 203, a secondresistor 205, a diode 204, and a NPN bipolar transistor 202 having anemitter, a base, a collector, and a plate. Additional details of theType A′ building block 201 can be as described earlier with reference toFIG. 7A.

FIG. 10A is a schematic block diagram of a pad circuit according to asecond embodiment. The illustrated pad circuit 282 includes a Type A′building block 201 connected in a cascade with a Type B′ building block231 between the pad 42 and the node 82. As described above, the Type A′building block 201 can be configured to have a trigger voltage V_(T)_(—) _(A′) equal to about the trigger voltage V_(T) _(—) _(B′) of theType B′ building block 231. However, the holding voltage V_(H) _(—)_(A′) of the Type A′ building block 201 can be configured to be greaterthan the holding voltage V_(H) _(—) _(B′) of the Type B′ building block231. Thus, the pad circuit 282 can be employed, for example, in an inputpad having a relatively moderate operating voltage and requiring arelatively moderate holding voltage. For example, if V_(T) _(—) _(A′)and V_(T) _(—) _(B′) are equal to about 9 V, V_(H) _(—) _(A′) is equalto about 5 V, and V_(H) _(—) _(B′) is equal to about 2.5 V, the padcircuit 282 can have a trigger voltage of about 18 V and a holdingvoltage of about 7.5 V. Thus, the pad circuit 282 can have a holdingvoltage and trigger voltage appropriate for the pad VH2 in Table 1.

FIG. 10B is a circuit diagram of the pad circuit of FIG. 10A. Theillustrated pad circuit 282 includes a Type A′ building block 201 and aType B′ building block 231 connected in a cascade configuration betweenthe pad 42 and the node 82. The Type A′ building block 201 includes afirst resistor 203, a second resistor 205, a diode 204, and a NPNbipolar transistor 202 having an emitter, a base, a collector, and aplate. Additional details of the Type A′ building block 201 can be asdescribed earlier with reference to FIG. 7A. The Type B′ building block231 includes a PNP transistor 232, a NPN bipolar transistor 233, a firstresistor 234, a second resistor 235, a third resistor 236, and a diode237. The PNP transistor 232 includes an emitter, a base, and acollector, and the NPN bipolar transistor 233 includes an emitter, abase, a collector and a plate. Additional details of the Type B′building block 231 can be as described earlier with reference to FIG.8A.

FIG. 11A is a schematic block diagram of a pad circuit according to athird embodiment. The illustrated pad circuit 283 includes two Type B′building block 231 connected in a cascade between the pad 42 and thenode 82. As described above, the Type B′ building block 231 can beconfigured to have a trigger voltage V_(T) _(—) _(B′) equal to about thetrigger voltage V_(T) _(—) _(A′) of the Type A′ building block 201 ofFIG. 7A. However, the holding voltage V_(H) _(—) _(B′) of the Type B′building block 231 can be configured to be greater than the holdingvoltage V_(H) _(—) _(A′) of the Type A′ building block 201. Thus, thepad circuit 283 can be employed, for example, in an input pad having arelatively moderate operating voltage and requiring a relatively lowholding voltage. For example, if V_(T) _(—) _(B′) is equal to about 9 Vand V_(H) _(—) _(B′) is equal to about 2.5 V, the pad circuit 283 canhave a trigger voltage of about 18 V and a holding voltage of about 5 V.Thus, the pad circuit 283 can have a holding voltage and trigger voltageappropriate for the pad VH3 in Table 1.

FIG. 11B is a circuit diagram of the pad circuit of FIG. 11A. Theillustrated pad circuit 283 includes two Type B′ building blocks 231connected in a cascade configuration between the pad 42 and the node 82.Each Type B′ building block 231 includes a PNP transistor 232, a NPNbipolar transistor 233, a first resistor 234, a second resistor 235, athird resistor 236, and a diode 237. The PNP transistor 232 includes anemitter, a base, and a collector, and the NPN bipolar transistor 233includes an emitter, a base, a collector and a plate. Additional detailsof the Type B′ building block 231 can be as described earlier withreference to FIG. 8A.

FIG. 12A is a schematic block diagram of a pad circuit according to afourth embodiment. The illustrated pad circuit 284 includes three TypeA′ building blocks 201 connected in a cascade between the pad 42 and thenode 82. The Type A′ building block 201 can be configured to have atrigger voltage V_(T) _(—) _(A′) equal to about the trigger voltageV_(T) _(—) _(B′) of the Type B′ building block 231 of FIG. 8A. However,the holding voltage V_(H) _(—) _(A′) of the Type A′ building block 201can be configured to be greater than the holding voltage V_(H) _(—)_(B′) of the Type B′ building block 231. Thus, the pad circuit 284 canbe employed, for example, in an output pad having a relatively highoperating voltage and requiring a relatively high holding voltage. Forexample, if V_(T) _(—) _(A′) is equal to about 9 V and V_(H) _(—) _(A′)is equal to about 5 V, the pad circuit 284 can have a trigger voltage ofabout 27 V and a holding voltage of about 15 V. Thus, the pad circuit284 can have a holding voltage and trigger voltage appropriate for thepad OVERVOLTAGE in Table 1.

FIG. 12B is a circuit diagram of the pad circuit of FIG. 12A. Theillustrated pad circuit 284 includes three Type A′ building blocksconnected in a cascade configuration between the pad 42 and the node 82.Each Type A′ building block 201 includes a first resistor 203, a secondresistor 205, a diode 204, and a NPN bipolar transistor 202 having anemitter, a base, a collector, and a plate. Additional details of theType A′ building block 201 can be as described earlier with reference toFIG. 7A.

FIG. 13A is a schematic block diagram of a pad circuit according to afifth embodiment. The illustrated pad circuit 285 includes two Type B′building blocks 231 connected in a cascade with a Type A′ building block201 between the pad 42 and the node 82. As described above, the Type A′building block 201 can be configured to have a trigger voltage V_(T)_(—) _(A′) equal to about the trigger voltage V_(T) _(—) _(B′) of theType B′ building block 231. However, the holding voltage V_(H) _(—)_(A′) of the Type A′ building block 201 can be configured to be greaterthan the holding voltage V_(H) _(—) _(B′) of the Type B′ building block231. Thus, the pad circuit 285 can be employed, for example, in anoutput pad having a relatively high operating voltage and requiring arelatively moderate holding voltage. For example, if V_(T) _(—) _(A′)and V_(T) _(—) _(B′) are equal to about 9 V, V_(H) _(—) _(A′) is equalto about 5 V, and V_(H) _(—) _(B′) s equal to about 2.5 V, the padcircuit 285 can have a trigger voltage of about 27 V and a holdingvoltage of about 10 V. Thus, the pad circuit 285 can have a holdingvoltage and trigger voltage appropriate for the pad UNDERVOLTAGE inTable 1.

FIG. 13B is a circuit diagram of the pad circuit of FIG. 13A. Theillustrated pad circuit 285 includes two Type B′ building blocks 231connected in a cascade with a Type A′ building block 201 between the pad42 and the node 82. The Type A′ building block 201 includes a firstresistor 203, a second resistor 205, a diode 204, and a NPN bipolartransistor 202 having an emitter, a base, a collector, and a plate.Additional details of the Type A′ building block 201 can be as describedearlier with reference to FIG. 7A. Each Type B′ building block 231includes a PNP transistor 232, a NPN bipolar transistor 233, a firstresistor 234, a second resistor 235, a third resistor 236, and a diode237. The PNP transistor 232 includes an emitter, a base, and acollector, and the NPN bipolar transistor 233 includes an emitter, abase, a collector and a plate. Additional details of the Type B′building block 231 can be as described earlier with reference to FIG.8A.

FIG. 14A is a schematic block diagram of a pad circuit according to asixth embodiment. The illustrated pad circuit 286 includes three Type B′building block 231 connected in a cascade between the pad 42 and thenode 82. As described above, the Type B′ building block 231 can beconfigured to have a trigger voltage V_(T) _(—) _(B′) equal to about thetrigger voltage V_(T) _(—) _(A′) of the Type A′ building block 201 ofFIG. 7A. However, the holding voltage V_(H) _(—) _(B′) of the Type B′building block 231 can be configured to be greater than the holdingvoltage V_(H) _(—) _(A′) of the Type A′ building block 201. Thus, thepad circuit 286 can be employed, for example, in an input pad having arelatively high operating voltage and requiring a relatively low holdingvoltage. For example, if V_(T) _(—) _(B′) is equal to about 9 V andV_(H) _(—) _(B′) is equal to about 2.5 V, the pad circuit 286 can have atrigger voltage of about 27 V and a holding voltage of about 7.5 V.Thus, the pad circuit 286 can have a holding voltage and trigger voltageappropriate for the pad VH4 in Table 1.

FIG. 14B is a circuit diagram of the pad circuit of FIG. 14B. Theillustrated pad circuit 286 includes three Type B′ building block 231connected in a cascade between the pad 42 and the node 82. Each Type B′building block 231 includes a PNP transistor 232, a NPN bipolartransistor 233, a first resistor 234, a second resistor 235, a thirdresistor 236, and a diode 237. The PNP transistor 232 includes anemitter, a base, and a collector, and the NPN bipolar transistor 233includes an emitter, a base, a collector and a plate. Additional detailsof the Type B′ building block 231 can be as described earlier withreference to FIG. 8A.

In the embodiments shown in FIGS. 9A-14B, cascaded building blockconfigurations employ Type A′ and Type B′ building blocks 201, 231.However, one or more additional building block types can be included.For example, a Type C′ building block having a holding voltage V_(H)_(—) _(C′) and a trigger voltage V_(T) _(—) _(C′) can be utilized. Thepad circuit 22 can combine i number of Type A′ building blocks, j numberof Type B′ building blocks, and k number of Type C′ building blocks suchthat the pad circuit 22 has a trigger voltage V_(TRIGGER) roughly equalto about i*V_(T) _(—) _(A′)+j*V_(T) _(—) _(B′)+k*V_(T) _(—) _(C′), and aholding voltage V_(HOLDING) roughly equal to about i*V_(H) _(—)_(A′)+j*V_(H) _(—) _(B′)+k*V_(H) _(—) _(C′). Providing additional typesof building block can increase the multitude of configurations of thecascade at the expense of an increase in design complexity.

FIG. 15 is a circuit diagram illustrating a pad circuit building blockin accordance with yet another embodiment. The Type C′ building block291 can be connected in a cascade with other building blocks between thepad 42 and the node 82. The illustrated Type C′ building block 291includes a first resistor 293, a second resistor 295, a diode 294, and aPNP bipolar transistor 292 having an emitter, a base, a collector, and aplate. The PNP bipolar transistor 292 can have a structure similar tothat of the PNP bipolar transistor 160 of FIG. 6C.

The diode 294 includes an anode electrically connected to the node 82,and a cathode electrically connected to the emitter of the PNP bipolartransistor 292 and to a first end of the first resistor 293 at a nodeN₅. The node N₅ can be electrically connected to another building blockin a cascade, such as the cascaded building blocks of FIGS. 4A and 4B,or to the pad 42. The first resistor 293 includes a second endelectrically connected to the base of the PNP bipolar transistor 292.The first resistor 293 can have, for example, a resistance between about11Ω and about 85Ω. In one embodiment, the first resistor 293 isimplemented using a multi-finger array to achieve the target resistance,such as an array of six fingers each having a resistance selected fromthe range of about 66Ω and about 510Ω. The second resistor 295 includesa first end electrically connected to the plate of the PNP bipolartransistor 292, and a second end electrically connected to the collectorof the NPN bipolar transistor 292 at a node N₆. The second resistor 295can have, for example, a resistance between about 200Ω and about 50 kΩ.The node N₆ can be electrically connected to another building block in acascade or to the node 82.

The pad circuit 22 can be, for example, any of the pad circuits 22 a-22p shown in FIG. 2, and the pad 42 can be any of the pads 42 a-42 p,including, for example, low-impedance output pads, high-impedance inputpads, and low-impedance power pads. The node 82 can be, for example, alow impedance node or pad of the power management IC 20 configured tohandle a relatively large shunted current. A transient signal event canbe received at the pad 42. If the transient signal event has a voltagethat is negative with respect to the node 82, the diode 294 can providecurrent which can aid in protecting the power management IC 20.

If the transient signal event has a voltage which is positive withrespect to the node 82, the PNP bipolar transistor 292 can aid inproviding transient signal protection. The trigger voltage of the TypeC′ building block V_(T) _(—) _(B′) can be based on the collector-emitterbreakdown voltage of the PNP bipolar transistor 292. The Type C′building block can have a holding voltage V_(H) _(—) _(C′) greater thaneither the holding voltage V_(H) _(—) _(A′) or V_(H) _(—) _(B′). Duringnormal operation, the absence of the LDD can make the leakage of the PNPbipolar transistor 292 low, even at relatively high temperatures. ThePNP bipolar transistor 292 can have a lower leakage current as comparedto a similarly sized PMOS transistor.

FIG. 16A is a schematic block diagram of a pad circuit according to aseventh embodiment. The illustrated pad circuit 297 includes a Type C′building block 291, a Type B′ building block 231, and a Type C′ buildingblock 291 connected in a cascade between the pad 42 and the node 82. Asdescribed above, the holding voltage V_(H) _(—) _(C′) of the Type C′building block 291 can be configured to be greater than the holdingvoltage V_(H) _(—) _(B′) of the Type B′ building block 231 or theholding voltage V_(H) _(—) _(A′) of the Type A′ building block 201.Furthermore, in certain processes, the leakage of the Type C′ buildingblock 291 can be less than that of the Type A′ and Type B′ buildingblocks 201, 231. Thus, the pad circuit 297 can be used, for example, ina very low leakage power pad having a relatively high operating voltageand requiring a relatively high holding voltage. For example, if V_(T)_(—) _(A′) and V_(T) _(—) _(B′) are equal to about 9 V, V_(T) _(—) _(C′)is equal to about 10 V, V_(H) _(—) _(B′) is equal to about 2.5 V, andV_(H) _(—) _(C′) is equal to about 10V, the pad circuit 285 can have atrigger voltage of about 29 V and a holding voltage of about 22.5 V.Thus, the pad circuit 297 can have a holding voltage and trigger voltageappropriate for the pad Vcc in Table 1. Additionally, in certainprocesses, the leakage current of the pad circuit 297 can be less thancertain pad circuit configurations using only Type A′ and Type B′building blocks, and thus pad circuit configurations with Type C′building blocks can be employed for very low leakage pads.

FIG. 16B is a circuit diagram of the pad circuit of FIG. 16A. Theillustrated pad circuit 297 includes a Type C′ building block 291, aType B′ building block 231, and a Type C′ building block 291 connectedin a cascade between the pad 42 and the node 82. Each Type C′ buildingblock 291 includes a first resistor 293, a second resistor 295, a diode294, and a PNP bipolar transistor 292 having an emitter, a base, acollector, and a plate. Additional details of the Type C′ building block291 can be as described earlier with reference to FIG. 15. The Type B′building block 231 includes a PNP transistor 232, a NPN bipolartransistor 233, a first resistor 234, a second resistor 235, a thirdresistor 236, and a diode 237. The PNP transistor 232 includes anemitter, a base, and a collector, and the NPN bipolar transistor 233includes an emitter, a base, a collector and a plate. Additional detailsof the Type B′ building block 231 can be as described earlier withreference to FIG. 8A.

FIG. 17A is a perspective view of one implementation of the pad circuitof FIG. 12B. The illustrated pad circuit 300 includes a bonding pad 305,a first Type A′ building block 301, a second Type A′ building block 302,and a third Type A′ building block 303 connected in a cascade. Thelayout of the first Type A′ building block 301 is configured such thatthe first Type A′ building block 301 can fit below the bonding pad 305.The second and Type A′ building blocks 302, 303 have layouts extendingoutside the bonding pad area.

During back-end fabrication (for example, fabrication of metal layers),building blocks can be included in a cascade configuration with thefirst Type A′ building block. Thus, for example, the pad circuit 300 canbe configured to have the configuration shown in FIG. 9B by changing themetal layers. Furthermore, additional building blocks, such as a Type B′building block can be placed adjacent to the pad 305, and can beincluded in the cascade by changing metal layers. Thus, an IC using thepad circuit 300, such as the power management IC 20, can be configuredfor a particular electronic system or application.

As will be described in further detail below with reference to FIGS.17B-17I, the pad circuit 300 can advantageously be constructed withthree metal layers, thereby permitting fabrication in processes withlimited numbers of metal layers. Moreover, the pad circuit 300 can beimplemented in a small circuit area, and a large portion of the padcircuit 300 can be positioned directly under the bonding pad 305.

FIG. 17B is a cross section of the pad circuit 300 of FIG. 17A takenalong the line 17B-17B. The first Type A′ building block 301 includes asubstrate 307, plates 309, a deep n-well 310, n-wells 311, contacts 312,a first metal layer 313, first vias 314, a second metal layer 315,second vias 316, a third metal layer 317, and passivation layer 318. Incontrast to the Type A′ building block 201 shown in FIG. 7B, the firstType A′ building block 301 is illustrated with back end processing. Thedeep n-well 310 and n-wells 311 can electrically isolate the first TypeA′ building block 301 from other building blocks, such as the second andthird Type A′ building blocks 302, 303. Additional details of the baselayers of the first Type A′ building block can be similar to thosedescribed earlier with reference to FIG. 7B.

FIG. 17C is a cross section of the pad circuit of FIG. 17A taken alongthe line 17C-17C. The second Type A′ building block 302 can be formed inthe same substrate 307 as the first Type A′ building block 301. Thesecond Type A′ building block 302 can include plates 309, a deep n-well310, n-wells 311, contacts 312, a first metal layer 313, first vias 314,a second metal layer 315, second vias 316, and a third metal layer 317.Additional details of the base layers of the second Type A′ buildingblock 302 can be similar to those described earlier with reference toFIG. 7B. Skilled artisans will appreciate that the geometries of firstType A′ building block 301 and the second Type B′ building block 302 canbe different. For example, the plates 309 of the first Type A′ buildingblock 301 can have different plate widths than the plates 309 of thesecond Type A′ 302, as can been seen in FIG. 17E.

FIG. 17D is a cross section of the pad circuit of FIG. 17A taken alongthe line 17D-17D. The third Type A′ building block 303 can be formed inthe same substrate 307 as the first and second Type A′ building blocks301, 302. The third Type A′ building block 303 can include plates 309, adeep n-well 310, n-wells 311, contacts 312, a first metal layer 313,first vias 314, a second metal layer 315, second vias 316, and a thirdmetal layer 317. Additional details of the third Type A′ building block303 can be as described earlier in connection with FIG. 7B.

FIG. 17E is a top plan view of the active and polysilicon layers of thepad circuit of FIG. 17A. FIG. 17F is a top plan view of the contact andfirst metal layers of the pad circuit of FIG. 17A. As shown in FIG. 17E,each of the building blocks 301-303 includes a plurality of rows ofemitters 320, 322 and a plurality of rows of collectors 321, when viewedfrom above. The rows of emitters 320, 322 and collectors 321 extendsubstantially parallel to one another. As shown in FIG. 17F, theemitters 320 on both of the peripheries of the pad circuit 300 can havea single row of contacts, while emitters 322 not on the peripheries ofthe pad circuit 300 and collectors 321 can have a double row ofcontacts.

The contacts of the emitters 320, collectors 321 and emitters 322 can bespaced so as to permit first, and second vias to be stacked, as shown inFIGS. 17F-17H. The n-diffusion resistors 323 can have a resistancesimilar to that described above with reference to FIG. 7A. Eachn-diffusion resistor 323 can have, for example, a width W_(R) of 0.7 μmand a length L_(R) of 9 μm.

As shown in FIGS. 17E-17F, a guard ring 325 can be connected through tworows of contacts. Additionally, a substrate guard ring 326 can becontacted with a double row of contacts. The plates 327 a and plates 327b can each have ten fingers, and each plate can have a plate length of,for example, about 0.5 μm. The plates 327 a can have a width of, forexample, about 615 μm, and the plates 327 b can have a width of, forexample, about 300 μm. The contact to diffusion overlap can be, forexample, about 2

FIG. 17G is a top plan view of the first metal layer 313 and first vialayer 314 of the pad circuit of FIG. 17A. Four rows of vias 340 can beprovided to contact the drains of NPN bipolar transistors. FIG. 17H is atop plan view of the first via layer 314, the second metal layer 315 andthe second via layer 316 of the pad circuit of FIG. 17A. FIG. 17I is atop plan view of the third metal layer 317 and the second via layer 316of the pad circuit of FIG. 17A.

Although FIGS. 17A-17I describe the construction and dimensions of oneparticular layout for a cascaded pad circuit, skilled artisans willappreciate that this example was for purposes of illustration. Padcircuit building blocks can be formed in a variety of ways, and can havedifferent circuit layouts depending on a variety of factors, including,for example, fabrication process and application of the pad circuit.

FIG. 18A is a perspective view of one implementation of the pad circuitof FIG. 11B. The illustrated pad circuit 400 includes a first Type B′building block 401 and a second Type B′ building block 402. The layoutof the first and second Type B′ building blocks 401, 402 is configuredsuch that the both Type B′ building blocks 401, 402 can fit below abonding pad, which has been omitted from FIG. 18A for clarity.Additional building blocks, such as a Type A′ building block, can beplaced adjacent to the bonding pad, and can be included in the cascade,for example, by a change metal layers. Thus, an IC using the pad circuit400, such as the power management IC 20, can be configured for aparticular electronic system or application.

FIG. 18B is a cross section of the pad circuit of FIG. 18A taken alongthe line 18B-18B. The first Type B′ building block 401 includes asubstrate 407, plates 409, a deep n-wells 410, n-wells 411, contacts412, a first metal layer 413, first vias 414, a second metal layer 415,second vias 416, a third metal layer 417, and passivation layer 418. Incontrast to the Type B′ building block 231 shown in FIG. 8B, the Type B′building blocks 401, 402 of FIG. 18B are illustrated with back endprocessing. The deep n-wells 410 and n-wells 411 can provideelectrically isolation of building blocks, such as between first andsecond Type B′ building blocks 401, 402, as well as electrical isolationof each building block from the substrate 407. Additional details of thebase layers of the first Type B′ building block can be similar to thosedescribed earlier in connection with FIG. 8B.

The foregoing description and claims may refer to elements or featuresas being “connected” or “coupled” together. As used herein, unlessexpressly stated otherwise, “connected” means that one element/featureis directly or indirectly connected to another element/feature, and notnecessarily mechanically. Likewise, unless expressly stated otherwise,“coupled” means that one element/feature is directly or indirectlycoupled to another element/feature, and not necessarily mechanically.Thus, although the various schematics shown in the Figures depictexample arrangements of elements and components, additional interveningelements, devices, features, or components may be present in an actualembodiment (assuming that the functionality of the depicted circuits isnot adversely affected).

Applications

Devices employing the above described schemes can be implemented intovarious electronic devices. Examples of the electronic devices caninclude, but are not limited to, consumer electronic products, parts ofthe consumer electronic products, electronic test equipment, etc.Examples of the electronic devices can also include memory chips, memorymodules, circuits of optical networks or other communication networks,and disk driver circuits. The consumer electronic products can include,but are not limited to, a mobile phone, a telephone, a television, acomputer monitor, a computer, a hand-held computer, a personal digitalassistant (PDA), a microwave, a refrigerator, an automobile, a stereosystem, a cassette recorder or player, a DVD player, a CD player, a VCR,an MP3 player, a radio, a camcorder, a camera, a digital camera, aportable memory chip, a washer, a dryer, a washer/dryer, a copier, afacsimile machine, a scanner, a multi functional peripheral device, awrist watch, a clock, etc. Further, the electronic device can includeunfinished products.

Although this invention has been described in terms of certainembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Moreover, the various embodiments described above can becombined to provide further embodiments. In addition, certain featuresshown in the context of one embodiment can be incorporated into otherembodiments as well. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

What is claimed is:
 1. An apparatus comprising: a substrate; a firstp-well in the substrate, wherein the first p-well is configured tooperate as a base of a first NPN bipolar transistor; a first p-type basecontact region in the first p-well; an n-type isolation structureconfigured to electrically isolate the first p-well from the substrate,and wherein the n-type isolation structure is electrically connected toa first node; a first n-type emitter region in the first p-well, whereinthe first n-type emitter region is configured to operate as an emitterof the first NPN bipolar transistor, and wherein the first n-typeemitter region and the first p-type base contact region are electricallyconnected to a second node; a first n-type collector region at leastpartially in the first p-well and spaced apart from the first n-typeemitter region, wherein the first n-type collector region is configuredto operate as a collector of the first NPN bipolar transistor; a firstconductive plate positioned adjacent a portion of the first p-wellbetween the first n-type emitter region and the first n-type collectorregion; and a resistor electrically connected between the firstconductive plate and the second node, wherein the first NPN bipolartransistor is configured to activate in response to an electrostaticdischarge (ESD) event received between the first and second nodes, andwherein the first conductive plate provides an electrical path forcurrent injection into the base of the first NPN bipolar transistorduring the ESD event.
 2. The apparatus of claim 1, further comprising abonding pad positioned adjacent the substrate, wherein the first p-welland the n-type isolation structure are confined within an area of thebonding pad.
 3. The apparatus of claim 1, wherein the resistor comprisesat least one of n-diffusion or polysilicon.
 4. The apparatus of claim 1,wherein the resistor has a resistance selected from the range of about200Ω to about 50 kΩ.
 5. The apparatus of claim 1, further comprising: afirst spacer adjacent a first side of the first conductive plate; asecond spacer adjacent a second side of the first conductive plateopposite the first side, wherein the first spacer and the second spacerare dielectric, and wherein a doping adjacent the first spacer, thesecond spacer, and the first conductive plate comprises p-type.
 6. Theapparatus of claim 5, wherein the doping adjacent the first spacer, thesecond spacer, and the first conductive plate does not include anylightly doped drain (LDD) regions.
 7. The apparatus of claim 1, furthercomprising: a first n-well positioned adjacent the first p-well, whereinthe first n-well is configured to operate as a base of a first PNPbipolar transistor; a first p-type emitter region in the first n-well,wherein the first p-type emitter region is configured to operate as anemitter of the first PNP bipolar transistor; and a first n-type basecontact region in the first n-well, wherein the first n-type basecontact region and the first p-type emitter region are electricallyconnected to the first node, wherein the first p-well is furtherconfigured to operate as a collector of the first PNP bipolartransistor.
 8. The apparatus of claim 7, wherein a first portion of thefirst n-type collector region is disposed in the first p-well, andwherein a second portion of the first n-type collector region isdisposed in the first n-well.
 9. The apparatus of claim 7, furthercomprising: a second p-well in the substrate, wherein the first n-wellis positioned between the first and second p-wells, wherein the n-typeisolation structure is further configured to electrically isolate thesecond p-well from the substrate, and wherein the second p-well isconfigured to operate as a base of a second NPN bipolar transistor; asecond p-type base contact region in the second p-well; a second n-typeemitter region in the second p-well, wherein the second n-type emitterregion is configured to operate as an emitter of the second NPN bipolartransistor, and wherein the second n-type emitter region and the secondp-type base contact region are electrically connected to the secondnode; a second n-type collector region at least partially in the secondp-well and spaced apart from the second n-type emitter region, whereinthe second n-type collector region is configured to operate as acollector of the second NPN bipolar transistor; and a second conductiveplate positioned adjacent a portion of the second p-well between thesecond n-type emitter region and the second n-type collector region,wherein the second conductive plate is electrically connected to thefirst conductive plate.
 10. The apparatus of claim 9, furthercomprising: a second p-type emitter region in the first n-well, whereinthe second p-type emitter region is configured to operate as an emitterof the second PNP bipolar transistor; and a second n-type base contactregion in the first n-well, wherein the second n-type base contactregion and the second p-type emitter region are electrically connectedto the first node, wherein the first n-well is further configured tooperate as a base of the second PNP bipolar transistor, and wherein thesecond p-well is further configured to operate as a collector of thesecond PNP bipolar transistor.
 11. The apparatus of claim 10, furthercomprising a bonding pad positioned adjacent the substrate, wherein thefirst p-well, the second p-well, the first n-well, and the n-typeisolation structure are confined within an area of the bonding pad. 12.The apparatus of claim 1, wherein the n-type isolation structurecomprises a deep n-well region and one or more n-wells.
 13. Theapparatus of claim 1, wherein the first conductive plate comprisespolysilicon.
 14. The apparatus of claim 13, further comprising an oxidelayer disposed between the first conductive plate and the first p-well.15. An apparatus comprising: a substrate; a first p-well in thesubstrate, wherein the first p-well is configured to operate as a baseof a first NPN bipolar transistor; a first n-type emitter region in thefirst p-well, wherein the first n-type emitter region is configured tooperate as an emitter of the first NPN bipolar transistor; a firstn-type collector region spaced apart from the first n-type emitterregion, wherein the first n-type collector region includes a firstportion disposed in the first p-well, wherein the first n-type collectorregion is configured to operate as a collector of the first NPN bipolartransistor; a first conductive plate positioned adjacent a portion ofthe first p-well between the first n-type emitter region and the firstn-type collector region; a first spacer adjacent a first side of thefirst conductive plate; a second spacer adjacent a second side of thefirst conductive plate opposite the first side, wherein the first spacerand the second spacer are dielectric, and wherein a doping adjacent thefirst spacer, the second spacer, and the first conductive plate consistsessentially of p-type; a first n-well positioned on a first side thefirst p-well, wherein the first n-well is configured to operate as abase of a first PNP bipolar transistor, and wherein the first n-typecollector region further includes a second portion disposed in the firstn-well; a first p-type emitter region in the first n-well, wherein thefirst p-type emitter region is configured to operate as an emitter ofthe first PNP bipolar transistor, wherein the first p-well is furtherconfigured to operate as a collector of the first PNP bipolartransistor; a second n-well positioned on a second side the first p-wellopposite the first side, wherein the second n-well comprises a firstguard ring.
 16. The apparatus of claim 15, further comprising: a thirdp-well in the substrate, wherein the third p-well is configured tooperate as a base of a second NPN bipolar transistor; a second n-typeemitter region in the second p-well, wherein the second n-type emitterregion is configured to operate as an emitter of the second NPN bipolartransistor; a second n-type collector region spaced apart from thesecond n-type emitter region, wherein the second n-type collector regionincludes a first portion disposed in the second p-well, and wherein thesecond n-type collector region is configured to operate as a collectorof the second NPN bipolar transistor; a second conductive platepositioned adjacent a portion of the second p-well between the secondn-type emitter region and the second n-type collector region; a thirdspacer adjacent a first side of the second conductive plate; a fourthspacer adjacent a second side of the second conductive plate oppositethe first side, wherein the third spacer and the fourth spacer aredielectric, and wherein a doping adjacent the third spacer, the fourthspacer, and the second conductive plate consists essentially of p-type;a third n-well positioned on a first side the second p-well, wherein thethird n-well is configured to operate as a base of a second PNP bipolartransistor, wherein the second n-type collector region further includesa second portion disposed in the third n-well; and a second p-typeemitter region in the third n-well, wherein the second p-type emitterregion is configured to operate as an emitter of the second PNP bipolartransistor, wherein the second p-well is further configured to operateas a collector of the second PNP bipolar transistor.
 17. The apparatusof claim 16, wherein the first guard ring comprises a deep n-well layer.18. The apparatus of claim 16, wherein the first guard ring and thefirst n-type emitter region are electrically connected.
 19. Theapparatus of claim 16, further comprising a fourth n-well positioned ona second side the second p-well opposite the first side, wherein thefourth n-well comprises a second guard ring.
 20. The apparatus of claim16, further comprising a resistor having a first end electricallyconnected to the first conductive plate and a second end electricallyconnected to the first n-type emitter region.
 21. The apparatus of claim20, wherein the resistor has a resistance selected from the range ofabout 200Ω to about 50 kΩ.